Donald Trump's campaign is spilling over into the Capitol's committee rooms as Democrats seek to capitalize on the GOP's increasingly strained relationship with the Republican front-runner.

Senate Republicans split over a referendum on Donald Trump’s Muslim ban on Thursday, with four GOP senators voting against language aimed at preventing religious litmus tests for people entering the United States. The nonbinding amendment to an unrelated nuclear terrorism bill was written by Senate Judiciary ranking member Patrick Leahy (D-Vt.). The provision stated it is the sense of the Senate that the U.S. “must not bar individuals from entering into the United States based on their religion."

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The amendment passed with significant GOP support, but not before highlighting internal Republican schisms over Trump's latest controversial policy prescription. In a brief interview, Sen. Ted Cruz (R-Texas), who voted against Leahy's amendment, accused Democrats of muddying debate on an important national security bill.

"Nuclear terrorism legislation is not the proper place for political games," said Cruz, who has refused to criticize Trump for his proposals during the presidential campaign, even though he disagrees with a ban on Muslim travel to the U.S.

Sen. Jeff Sessions (R-Ala.), who speaks to Trump on occasion, called Leahy’s proposal “radical” and railed against it for about 30 minutes. He was joined in dissent by Cruz andRepublican Sens. David Vitter of Louisiana and Thom Tillis of North Carolina.

“I believe he means well,” Sessions said of Leahy. “But with regard to immigration, it is our responsibility to protect the rights and well-being first of the American citizens.”

GOP Sens. John Cornyn of Texas, Chuck Grassley of Iowa, Orrin Hatch of Utah, Lindsey Graham of South Carolina, Mike Lee of Utah, Jeff Flake of Arizona and David Perdue of Georgia supported the amendment. Several Democrats grinned visibly when Cruz’s proxy vote of “no” was cast, with one uttering into a hot mic: “Ohhh yeah.”

Leahy said afterward that he was "amazed" that four Republicans voted against barring religious tests, adding that he believes such a standard for entry could have prevented his grandparents from getting into the country.

"I just wanted to point out that we could show to the rest of the world that we don't have religious tests," Leahy said. "I assumed this would be a no-brainer."

Sessions posited that the amendment could make it more difficult to allow a moderate Muslim cleric into the country over a more radical cleric and also make it tougher for Yazidi and Christian minorities from the Middle East to seek refuge in the United States.

Sessions was clearly prepared for Leahy’s proposal, reading off prepared remarks for long periods during a committee hearing.

“I regret talking so long on this subject. But I really think religion in America is extremely important, and we need to be very careful how we act on it,” Sessions said.

For other Republicans, the vote was more complicated. Tillis said he was fine with the Leahy language if it weren't part of a national security bill. Now that it's been appended to the terrorism legislation he worries that the larger bill will become a Trump referendum.

He also knocked Leahy for meddling with a bill that he ended up voting against. Tillis voted for the terrorism bill.

"The comments of the presidential election are starting to influence the discussion in the committee," Tillis said. "We need to get that bill to the floor and we're adding things that are inspired by what people watch in TV at night versus focusing on that underlying bill."

Perdue said it wasn't hard for him to vote for Leahy's provision but added of the episode: "I didn't think it was necessary. It was just something we had to deal with."

Trump’s radical proposal has presented problems for Republicans all week. Many have denounced his plan, like Senate Majority Leader Mitch McConnell and House Speaker Paul Ryan. Yet most Republicans still say they would back him if he were to become the GOP nominee. Cruz, in particular, has carefully chosen not to directly battle with Trump.

Trump: 'I'm doing good for the Muslims'

During an interview with CNN's Don Lemon on Wednesday, Donald Trump defended his Muslim ban plan.

It was clear Democrats were aiming to tie Trump to congressional Republicans with the vote, as Sens. Dianne Feinstein (D-Calif.) and Dick Durbin (D-Ill.) repeatedly brought up his comments this week while touting the importance of Leahy's amendment, which would carry no force of law.

“You can’t ignore what a leading candidate says,” Feinstein said. “This is the first time we have an opportunity to say to this candidate: 'This is not what our nation stands for.'”